Each optical device produced generally has a set of prisms in each channel to reorient the otherwise upside-down image. The precision required in the placement and orientation of these prism sets has presented a problem. There is no manufacturing technology which can easily produce a prism set housing that conforms accurately enough to a detailed shape specification to satisfy the required accuracy of prism set orientation within the optical channel.
The accuracy of one candidate prism set housing production method, injection molding, is limited principally by two factors. The first factor is the machining of the mold. There is a limit, at the level of about 12 .mu.m (0.5 mil), to the accuracy of machining an article. The second factor is the shrinkage of the molded material. The resins commonly used for injection molding tend to shrink while cooling down from the injection temperature. The temperature and pressure at which the molding takes place affects the amount of shrinkage. Unfortunately, the more complex the molded shape the more difficult it is to predict the amount of shrinkage across a specified cut of the molded piece.
Because of the difficulty of precisely manufacturing a prism set housing to a specification, it is typical for the prism set to be held in place by an adjustable housing. This housing is secured into an optical device channel that is equipped with an aperture through which a technician may insert tools to adjust the orientation of the prisms. The prism set housing is adjusted until the prisms are oriented to within the required accuracy.
There are two problems with this type of procedure. The first problem is that it is time consuming and expensive to adjust the prism set orientation in each channel of every optical device produced. The second problem is that the aperture for tool insertion reduces the integrity of the optical channel. This is true even when efforts have been made to carefully reseal the tool insertion aperture. The reduction in optical channel integrity reduces the degree to which the device can be made watertight.